South Sudan in July became an independent country as part of an agreement reached with Washington's help in 2005. Border conflicts, ethnic tensions and disputes over oil continue to threaten the peace, however.
Victoria Nuland, a spokeswoman for the U.S. State Department, said Washington was "alarmed" by reports that Sudanese military forces bombed South Sudanese targets in recent days.
"Such incidents are unacceptable and threaten to escalate tensions between the two states," she said in a statement. "Additionally, continued aerial attacks on civilian targets by the Sudan armed forces are deplorable and constitute violations of international law for which there must be accountability."
She warned that both sides were moving closer to "greater conflict" and away from the commitments that ended Sudan's bloody civil war in 2005. Both sides, she said, are "inflaming conflict" in the region.
U.S. Rep. Frank Wolf, R-Va., co-chairman of the Congressional Human Rights Caucus, said in a 16-page field report following his visit to the region the crisis in Sudan is growing more severe.
He said "thousands" of people were streaming across the border into refugee camps setup in South Sudan. Most of the refugees there are women and children, he said, because most of the men have been killed in recent fighting.